Fluoroapatite has almost the same crystalline structure as hydroxyapatite, and therefore shows almost the same adsorption characteristics (adsorption ability) to proteins as hydroxyapatite.
Further, fluoroapatite is a substance that is stabler than hydroxyapatite due to fluorine atom thereof, and therefore has high acid resistance. For these reasons, fluoroapatite has advantages in that it has high resistance to acid solutions, and therefore is capable of separating a protein in an acid solution.
Such fluoroapatite is generally synthesized by adding (mixing) ammonium fluoride as a fluorine source into (with) a slurry containing hydroxyapatite (JP A-2004-330113 is an example of related art.).
However, in the fluoroapatite synthesized by such a method, ammonia which is derived from the ammonium fluoride is adsorbed thereto as an impurity during the synthesizing process. Apatites such as fluoroapatite have a high ability to adsorb ammonia, and therefore in a case where a slurry containing synthesized fluoroapatite is spray-dried (granulated) to obtain particles, ammonia remains in the particles (fluoroapatite particles). That is, it is very difficult to remove ammonia from the fluoroapatite particles.
Therefore, an amount of ammonia remaining in the thus synthesized fluoroapatite particles is different from lot to lot, which makes it difficult to obtain fluoroapatite particles having uniform characteristics.
Further, ammonia adsorbed to hydroxyapatite particles inhibits substitution of hydroxyl groups of hydroxyapatite by fluorine atoms of ammonium fluoride molecules, and therefore there are also problems in that a ratio of substitution of hydroxyl groups of hydroxyapatite by fluorine atoms can be increased only to a certain extent, and therefore it cannot be expected that acid resistance of the synthesized fluoroapatite is further improved.
Furthermore, since fluoroapatite is often used for separating a protein in an acid solution, it is preferred that an amount of the protein to be adsorbed to fluoroapatite is large so that a large amount of the protein can be separated from a sample (that is, the acid solution containing the protein). From this viewpoint, fluoroapatite having a large specific surface area is preferably used.
For these reasons, fluoroapatite which contains an impurity such as ammonia at a low level and has superior acid resistance and a large specific surface area can be preferably used for separating a protein (particularly, contained in an acid solution). However, a method of producing such fluoroapatite has not yet been developed.